1. Field of the Invention
The present invention generally relates to a method and apparatus for anodizing the surface of a metallic cylinder, and more particularly to an improved method and apparatus for anodizing the surface of a bimetallic cylinder, such as an aluminum clad copper pin-piston, for use in a thermal conduction module.
2. Description of the Prior Art
U.S. Pat. No. 4,226,281 to Richard Chu entitled "Thermal Conduction Module" and assigned to the assignee of this application, discloses a module for transferring heat from semiconductor chips in which spring-loaded pin-pistons in contact with the chip transfer heat to housing referred to in the art as a piston hat. Commonly, aluminum pin-pistons are used in combination with an aluminum piston hat, and the pin-pistons are isolated electrically from the piston hat by anodizing the surface of the pin- piston or the surface of the piston hat that can contact the pin-piston. An anodized aluminum layer can provide good electrical isolation without significant degradation of heat transfer between the pin-piston and the piston hat.
Copper is a desirable material from which to fabricate pin-pistons and piston hats, owing to its greater coefficient of thermal conductivity as compared with that of aluminum. However, as a practical matter, it is difficult to electrically insulate copper pin-pistons from a copper pin hat.
Aluminum clad copper pin-pistons and pin hats are advantageous in that they provide a heat transfer coefficient comparable to copper and can be electrically isolated by anodizing the aluminum cladding surface, preferably the surface of the pin-piston. However, conventional anodizing processes cannot be used to anodize fabricated pin-pistons because the exposed copper ends of the pin-piston cannot be immersed in the anodizing electrolyte bath. Masking of the part to obtain selective anodization is not desirable because of cost and damage caused by handling.